Potassium polymetaphosphate, also known as potassium Kurrol's salt or potassium metaphosphate, is a polymeric metaphosphate of the formula (KPO.sub.3).sub.n, where n is the degree of condensation and is usually much greater than 100. The Kurrol salts are generally made by dehydrating monopotassium orthophosphate, resulting in a straight chain polyphosphate with a high degree of polymerization. The degree of polymerization can range from several hundred to several million phosphorous atoms per chain, depending upon the conditions under which the chemical reaction is carried out.
A survey of the chemistry of potassium polymetaphosphate is provided by J. R. Van Wazer in Phosphorous And Its Compounds, Interscience Publishers, Inc., New York, a division of John Wiley and Sons, Inc., Vol. I, (1958), pages 665-678, which is incorporated herein by reference.
Potassium polymetaphosphate has been used in the food industry as an emulsifier and as a moisture retaining agent, particularly in connection with ground meat products (See U.S. Pat. No. 2,852,392).
U.S. Pat. No. 2,655,212 discloses aqueous solutions or dispersions of potassium Kurrol salt and one or more solubilizing agents which are reacted with aqueous dispersions containing milk protein. Added to milk, the resulting polymer/milk protein gels are disclosed to be useful as thickening or stabilizing agents, particularly in chocolate milk drinks.
Potassium polymetaphosphate has also been evaluated for use as a builder for laundry detergent products. H. Stupel (Soap Factory, Hochdorf, Switzerland, Textile-Prayis 7,231-4) (1952). The detergenty of various soap and phosphate-builder combinations containing high levels of phosphates (0.5 g-6.0 grams of phosphate-builder with 1-2 grams of soap) were tested.
Potassium polymetaphosphate has a very low solubility in water but good solubility in dilute solutions of salts with singly charged cations other than potassium.
The dissolution of potassium polymetaphosphate in a solution containing monovalent cations other than potassium occurs via an ion exchange process. Singly charged cations replace the potassium ion in the dispersed, long chained, linear molecules of (KPO.sub.3).sub.n. An excess of the singly charged cations causes the formation of a gelatinous mass which upon dilution gives a viscous solution. The ion-exchange reaction which occurs in the aqueous media thus results in the solubilization of the complex phosphate compound and produces clear gels. The gels are not stable since significant hydrolysis of the polymer occurs over relatively short times (typically under two hours) resulting in a substantial loss of viscosity.